The major tungsten mineral in many tungsten ores is scheelite, CaWO.sub.4. Scheelite ores have been treated by gravity techniques and froth flotations to produce scheelite concentrates. Gravity techniques work reasonably well for coarse grained scheelite and froth flotation must be resorted to if fine grain scheelite is present in the ore. Concentration of scheelite by froth flotation provides acceptable recoveries but difficulties are encountered in the production of high grade concentrates with acceptable recoveries.
Scheelite in scheelite ores is generally disseminated in host rocks having as their major constituents silicate and calcareous minerals. Typical silicate minerals include pyroxene, mica, amphibole, garnet and epidote. Typical calcareous minerals include calcite, apatite, collophane and fluorite. Sulfide minerals are also commonly present in scheelite ore and are, in the absence of special precautions, rendered flotable to some extent by the same collectors used for scheelite. Substantial amounts of these silicate and calcareous minerals are floated with the scheelite thereby lowering the grade of the scheelite concentrate. Although the silicate and calcareous minerals can be depressed by suitable conditioners, scheelite recovery frequently suffers because some of the scheelite is depressed along with the silicate and other calcareous minerals.
Attempts have been made to solve this dilemma of sacrificing grade for recovery or recovery for grade. For example, Mercade in U.S. Pat. No. 3,915,391 discloses a process for recovering a scheelite concentrate from a low grade scheelite ore. Finely divided scheelite ore is conditioned at a high pH using intense agitation in the presence of predetermined quantities of freshly prepared metal sols and in the presence of starvation amounts of a fatty acid collector. The thus-conditioned pulp is subjected to froth flotation to recover a froth of the scheelite concentrate. Although this process works reasonably well, the necessity for using freshly prepared metal sols increased the complexity and costs of the overall process.